1. Field of the Invention
The present invention relates to a reactor for performing reactions in a fluidized bed operating under countercurrent gas-solid flow conditions and processes for performing reactions using the reactor.
2. Discussion of the Background
A fluidized bed is formed by passing a fluid, usually a gas, upwards through a bed of solid particles. Fluidization is achieved if the gas superficial velocity is higher than a so-called "Minimum buoyancy velocity," that is mainly a function of the density and dimensions of the particles involved.
Gas velocities higher than minimum buoyancy first induce bubbling, then cause catalyst entrainment out of the bed.
Many papers and books deal with the theoretical basis of fluidization ("Fluidization and Fluid-Particle Systems", F. A. Zenz, D. F. Othmer--Reinhold Publishing Corporation, 1960; "Gas Fluidization Technology", Edited by D. Geldart--John Wiley & Sons, 1986; "Fluidization Engineering", 2nd Edition, K. Kunii, O. Levenspiel--Butterworth-Heinemann, 1991).
The earliest applications of fluidization were for the purpose of carrying out chemical reactions. For example, the first industrial application was the gasification of coal (1926), a non-catalytic gas-solid reaction.
The widespread use of fluidized beds as chemical reactors began in the early forties with the construction of the first fluidized bed catalytic cracker (FCC). In that case, the fluidized bed technology was used to perform a catalytic reaction, i.e., the solid particles served as catalyst, undergoing no chemical changes and limited physical changes during the course of the reaction.
The catalytic materials for fluidized bed reactors are manufactured to combine favorable chemical characteristics with suitable physical properties (particle size, shape, density, resistance to attrition, etc.).
Some features of fluidized beds make them more attractive than fixed beds and other types of gas-solid reactors for performing many chemical reactions.
The main advantages of fluidized beds are:
Temperature uniformity, so that "hot spots" are easily avoided. For endothermic reactions, heat can be transferred to the reactants by the hot regenerated catalyst, avoiding heat transfer through walls; for exothermic reactions, a heat exchanger system can be provided inside the reactor vessel, which efficiently removes reaction heat owing to the fluid bed properties. PA1 Ease of solids handling: fluidized solids can be continuously added and/or removed from the system. In catalytic reactors, deactivated solids may be transferred to a second fluidized bed system, regenerated and then recycled to the reactor. PA1 Scale of operations: successful operations have been achieved with columns as large as 30 m internal diameter; PA1 Turndown capacity: the gas flow rate can be varied over a wide range; PA1 Pressure drop: the pressure drop through a fluidized bed of solids is much less than for the same bed at the same gas velocity under fixed bed conditions, especially for fine particles. PA1 Substantial backmixing of solids and gas, often resulting in lower conversions and selectivities than with most competing types of reactor; PA1 By-passing of gas via bubbles or jets which causes gas-solid contact to be less effective, leads to a further lowering of conversions and may also contribute to poor selectivity; PA1 Entrainment of solids which can lead to loss of expensive materials as well as to pollution of the atmosphere.
On the other hand, fluidized beds have certain limitations and disadvantages that have been clearly recognized, including:
These limitations can be lessened by improving the design of the reactor. One effective modification is the insertion in the reactor vessel, at various heights, of internal structure in the form of horizontal or vertical baffles.
These baffles can assume many physical forms (bundles of tubes, wire cloth, perforated plates, triangular tiles, etc.).
The main role of the baffles is to drastically reduce the phenomena of backmixing, hindering the free movement of solids along the bed. The main fluidized bed is thus divided into a number of smaller beds, each one behaving like a stand-alone bed, exchanging gas and solids with the neighboring beds.
Baffles help to reduce by-passing, induce breakage of the largest gas bubbles, and reduce entrainment as well because the solids, colliding with the baffles lose part of their kinetic energy. Industrial applications of fluidized beds with baffles have been described (See publications above cited).